This proposal aims to determine function and regulation of a mitotic molecular motor protein called kinesin-5 in vertebrate neurons during their normal development. Central nervous system vertebrate neurons typically possess one long thin axon with a uniformly plus-end-distal microtubule array and several short thick tapering dendrites with mixed polarity microtubule array. While the axon?s main purpose is to generate an outgoing impulse to a potentially faraway target, dendrites function to gather and consolidate incoming information. Kinesin-5, a homotetrameric motor protein known to be essential for the formation of the mitotic spindle, has recently been shown to be present and functional in neurons. Also called Eg5 or kif11, kinesin-5 belongs to a group of motor proteins that are crucial for cell division by influencing the transport of microtubules relative to one another. This proposal hypothesizes a unique role for kinesin-5 within the terminally post-mitotic neuron and aims to answer key questions on the regulation of kinesin-5?s distribution and activity. A neuron where kinesin-5 is experimentally depleted or inhibited displays faster growing axons and abnormally long and thin dendrites. Therefore, the overarching hypothesis of this proposal is that kinesin-5 acts as a ?brake? to modulate the growth and development of these neurites. Mechanistically, the hypothesis is that kinesin-5 attenuates the growth of axons and dendrites by limiting the transport of microtubules into them by other molecular motors. Previous work was done on axons; the emphasis here is on dendrites. Regulation of kinesin-5 is explored through three different specific aims ? the first aimed to determine kinesin-5 function in development and maintenance of the dendritic microtubule array; the second aimed to determine the regulatory role of kinesin-5 phosphorylation of threonine residue 926; and the third aimed to determine whether kinesin-5 recognizes specific microtubule post-translational modifications in order to become enriched in dendrites as they mature. Proposed studies in these three aims utilize cutting edge cell biological analyses including live-cell imaging techniques, quantitative immunofluorescence and sophisticated RNA interference approaches. The hypothesis that molecular motors strongly influence dendritic characteristics through regulation of microtubule polarity patterns may have profound implications for the progression and potential treatments of diseases of the nervous system.